The invention concerns distance measuring instruments. Distance measuring instruments traditionally have used microwaves and infrared radiation to measure distance by way of the velocity of light and time of travel. With the advent of low cost visible laser diodes, instruments which project a visible beam are very popular in this application. In addition to the obvious cost advantage, the operator is able to see the measuring spot because of the visible light beam. Typically, a one milliwatt beam a few millimeters in diameter is used for this purpose in such an instrument. Examples are the model DLE 30 manufactured by Bosch or the DISTO manufactured by Leica of Switzerland. These instruments may be hand held, placed on the floor or used with a tripod. In measuring horizontal distance, the operator adjusts the instrument crudely to be approximately horizontal, which is satisfactory for most cases. The error which results is the cosine of the angle between true horizontal and the actual instrument angle. This error times the distance measured is usually small enough to ignore.
There is, however, a frequent need to use the horizontal beam as a level reference. Normally a separate instrument and setup are used for that purpose. By combining the distance measurement and the projection of a truly horizontal beam, the operator saves time by doing the setup just once. In addition, the measurement is more accurate, which in some cases can be important.
The object of this invention is to provide a truly horizontal measuring beam even though the instrument is not horizontal, such that the instrument can be used to establish level as well as to measure distance.